In recent years, attempts have been made to improve throughput by providing a plurality of antennas on both a base station apparatus (referred to simply as a base station hereinafter) and a terminal apparatus (referred to simply as a terminal hereinafter) to introduce the MIMO (multiple-input multiple-output) communication technology on uplink. In this MIMO communication technology, a study is made on data transmission using precoding control in a terminal. In the precoding control, the base station estimates a channel condition between the base station and the terminal, from a receiving condition of a reference signal (Sounding Reference Signal: SRS) independently transmitted from each antenna of the terminal, selects a precoder which is optimal for the estimated channel condition and applies the precoder to data transmission.
Particularly, precoding control based on a transmission rank is applied to LTE-Advanced (Long Term Evolution-Advanced: hereinafter, referred to as a LTE-A). Specifically, the base station selects the most suitable rank and precoder for the channel matrix formed by the values of observed SRSs transmitted from the terminal. Herein, a rank refers to the number of space multiplexing (the number of layers) in space division multiplexing (SDM) and is the number of independent data transmitted at the same time. To be more specific, code books having different sizes are employed for respective ranks. The base station receives a reference signal transmitted from the terminal, estimates a channel matrix from the received signal, and selects a rank and a precoder which is optimal for the estimated channel matrix.
In a communication path such as mobile communication, having a relatively large channel variation, a hybrid automatic repeat request (HARQ) is applied for an error controlling technique. HARQ is a technique whereby the transmitting side retransmits data, and the receiving side combines the received data and the retransmitted data to improve error correction performance and achieve high quality transmission. As a HARQ method, adaptive HARQ and non-adaptive HARQ are under study. Adaptive HARQ is a method for allocating retransmitted data to any resource. On the other hand, non-adaptive HARQ is a method for allocating retransmitted data to predetermined resources. In an uplink of LTE, the non-adaptive HARQ scheme is employed among HARQ schemes.
A non-adaptive HARQ scheme will be described with reference to FIG. 1. In non-adaptive HARQ, the base station determines resources for allocating data in the first data allocation. The base station then reports transmission parameters to a terminal through a downlink control channel (PDCCH: Physical Downlink Control Channel). The transmission parameters include information such as allocated frequency resources indicating information on resource allocation, a transmission rank number, a precoder, and a modulation scheme/a coding rate. The terminal acquires the transmission parameters transmitted through the PDCCH and transmits first data, using a predetermined resource in accordance with the aforementioned resource allocation information.
The base station receives the first data and reports, to the terminal, a NACK corresponding to data which could not be demodulated in the first data, through a HARQ reporting channel (PHICH: Physical Hybrid-ARQ Indicator Channel). The terminal receives the NACK and controls retransmission by using the transmission parameters reported through the PDCCH, the parameters including information resource allocation and the like. Specifically, the terminal generates and transmits retransmission data, using an allocation frequency resource, a precoder, a modulation scheme, and the like, which are the same as those in the first transmission. The terminal changes an RV (Redundancy Version) parameter depending on the number of retransmission requests. The RV parameter represents a reading position in a memory (referred to as a circular buffer) for storing Turbo-coded data. For example, when the memory is equally divided into approximately four regions and tops of the areas are assigned zero, one, two, and three respectively, the terminal changes an RV parameter (a reading position) in order of zero, two, one, three, and zero depending on the number of retransmission requests.
Non-adaptive HARQ is often used together with Synchronous HARQ employing the constant transmission interval. In LTE, retransmission data is retransmitted eight subframes after the report of the NACK.
Non-adaptive HARQ is performed on a per predetermined control unit basis, the control unit is referred to as a code word (CW). The CW is a control unit to which the same modulation scheme and coding rate are applied. As with the CW processed in a physical layer dealing with modulation and coding, the control unit may be referred to as a transport block (TB) since the control unit is processed in a MAC layer dealing with HARQ, and the CW may be distinguished from the TB. The present embodiment however employs uniform notation “CW” without a distinction therebetween hereafter.
In LTE, the transmission of one CW is generally applied to rank 1 (in transmission in a single rank) in the first transmission, and the transmission of two CWs is applied to ranks 2, 3, and 4 (in transmission in multiple ranks) in the first transmission. In the transmission in multiple ranks, CW0 is allocated to Layer 0 and CW1 is allocated to Layer 1 in rank 2. In rank 3, CW0 is allocated to Layer 0, and CW1 is allocated to Layer 1 and Layer 2. In rank 4, CW0 is allocated to Layer 0 and Layer 1, and CW1 is allocated to Layer 2 and Layer 3.
When retransmitting only CWs allocated to a plurality of layers, the terminal transmits one CW at a time in rank 2. To be more specific, when retransmitting CW1 in rank 3 and CW0 or CW1 in rank 4, the terminal transmits these CWs as one CW in rank 2.
Since the base station includes a larger number of antennas compared to the terminal, the base station is flexibly installed relatively. For this reason, a so-called multiuser MIMO, which assigns the same resource to a plurality of terminals, can be applied through an adequate process on a received signal in the base station. An example case will be described where the same resource is allocated to two terminals through the terminal having one transmitting antenna and the base station having two receiving antennas. This case can be equivalently treated as a MIMO channel with two transmitting antennas and two receiving antennas, and the base station can process a received signal. To be more specific, the base station performs a general MIMO received-signal process such as spatial filtering, canceller, and maximum likelihood estimation, thereby detecting respective signals transmitted from a plurality of terminals. With multiuser MIMO, the base station estimates interference values between terminals based on the channel condition between the base station and each terminal, and sets transmission parameters for the respective terminals by considering interference values, in order to more stably operate a communication system.
As described above, a MIMO operation for a single terminal (a single user) provided with a plurality of antennas is sometimes referred to as a single-user MIMO to distinguish it from the multiuser MIMO. An operation to allocate a plurality of terminals, each of which is capable of a single-user MIMO operation and has more than one transmission antennas provided thereon, to the same resource is also referred to as a multiuser MIMO.
The terminal transmits not only the SRS described above but also a demodulation reference signal (Demodulation RS or DMRS) to the base station, and the base station uses the received DMRS for demodulating data. In LTE-A, the DMRS is transmitted for each layer. The terminal transmits the DMRS using the same precoding vector as that of the signal transmitted for each layer. In order for a plurality of terminals to transmit the DMRSs for a plurality of layers in the same frequency resource, some multiplexing process is needed. In LTE-A, as a process of multiplexing the DMRSs, multiplexing using an orthogonal cover code (OCC) is used in addition to multiplexing using a cyclic shift sequence used in LTE to multiplex a plurality of terminals.
The cyclic shift sequence is generated by cyclic shift of a predetermined one of CAZAC (constant amplitude zero auto-correlation) sequences having good auto-correlation characteristics and a constant amplitude. For example, twelve cyclic shift sequences each of which having a starting point at one of twelve points that equally divide a CAZAC sequence along the code length are used. In the following, the starting point will be expressed as nCS.
As for the OCC, spreading codes having a sequence length of 2 are formed using a DMRS, which includes two symbols per sub-frame, taking into consideration the transmission format of the uplink data. To be more specific, in LTE-A, as OCCs, two spreading codes having a sequence length of 2, {+1, +1} and {+1, −1}, are formed. In the following, a spreading code according to the OCC will be expressed as nOCC. For example, the two spreading codes {+1, +1} and {+1, −1} are expressed as nOCC=0 and 1, respectively.
Further, nCS and nOCC are included in transmission parameters reported from the base station to the terminal through the PDCCH. A specific method of reporting the transmission parameters including nCS and nOCC, in particular, a specific reporting method using the single-user MIMO, will be described later.
Next, interference between DMRSs multiplexed in the same frequency resource will be described. FIG. 2 is a schematic diagram showing interference between DMRSs to which nCS=6 and nOCC=0 are allocated. The interference between the DMRSs formed by the cyclic shift sequence and the OCC described above is characterized in that the DMRSs having the same value of nOCC and adjacent values of nCS interfere with each other. For example, reference signals having the same value of nOCC and adjacent values of nCS that differ from each other by up to 3 or so (indicated by the arrows in FIG. 2) (that is, reference signals whose nOCC is 0 and whose nCS falls within a range of 3 to 5 or a range of 7 to 9 in FIG. 2) interfere with each other. Therefore, as for nCS, in order for reference signals to be allocatable at the same time, the values of nCS of the reference signals preferably differ by 6 or so.
As for nOCC, on the other hand, if reference signals to be allocated (to be multiplexed) at the same time have the same code length, that is, the same bandwidth allocated thereto, the reference signals are expected to be orthogonal to each other if they have different values of nOCC. The degree of the orthogonality (referred to simply as orthogonality) depends on the fading correlation between the two symbols in one sub-frame to which the reference signals (DMRSs) are allocated. For example, in a low-speed moving environment, which is a primary application of MIMO, high orthogonality is expected to be assured.
Next, a method of reporting a spreading code of a DMRS in the single-user MIMO will be described. According to a method of reporting a spreading code of a DMRS in LTE, the base station sets arbitrary spreading codes using a parameter nDMRS(1) set for each user in a higher layer assuming a relatively long period and a parameter nDMRS(2) that is a transmission parameter reported through the PDCCH and set for a relevant transmission sub-frame by decision of the scheduler, and indicates the spreading codes to the terminal. The terminal generates a DMRS using a prescribed nCS calculated from the indicated parameter (nDMRS(1) or nDMRS(2)).
In LTE-A, there is proposed a method of expanding the reporting method described above to the single-user MIMO (see Non-Patent Literature 1, for example). In Non-Patent Literature 1, the starting point of the cyclic shift sequence and the set value of OCC for the k-th layer (k=0 to 3) are set as nDMRS,k(2) (corresponding to nCS described above) and nOCC,k, respectively. In Non-Patent Literature 1, information reported through higher layers or the PDCCH is only the set values (nDMRS,0(2) and nOCC,0) for the 0-th layer (k=0, Layer 0), and the set values for the remaining layers (k=1 to 3, Layers 1 to 3) are determined by calculation from the set values for the 0-th layer (k=0, Layer 0). This is an attempt to minimize the overhead involved in reporting of the controlling signal.
To be more specific, Non-Patent Literature 1 discloses that each set value is set as follows in order to avoid the interference between the reference signals as far as possible in the single-user MIMO.
Specifically, nDMRS,0(2) is defined as (nDMRS,0(2)+Δk) mod 12
where
in transmission using two layers, Δk=0 for k=0, and Δk=6 for k=1,
in transmission using three layers, Δk=0 for k=0, Δk=6 for k=1, and Δk=3 for k=2, or
Δk=0 for k=0, Δk=4 for k=1, and Δk=8 for k=2, and
in transmission using four layers, Δk=0 for k=0, Δk=6 for k=1, Δk=3 for k=2, and Δk=9 for k=3.
Further, nOCC,k is defined as nOCC,0 or (1−nOCC,0)
where
nOCC,k=nOCC,0 for k=1, and nOCC,k=(1−nOCC,0) for k=2 or 3.